1. Field of the Invention:
The present invention generally relates to communications systems, and, more particularly, to interference reduction circuits to provide polarization isolation is frequency reuse systems of the type where each "polarization" of a dual-polarized communications link carries independent information at the same RF frequencies.
2. Description of the Prior Art:
Using two orthogonal polarizations simultaneously effectively doubles the bandwidth of a communications system. Even greater effective bandwidth can be achieved by using multiple antenna beams at the satellite which are isolated via spatial separation among the beams. Systems using these techniques are referred to as frequency reuse sytems and are of particular interest in communications satellite systems. However, due to the polarization characteristics of the spacecraft antenna and the earth station antenna such as non-ideal feed axial ratio, antenna depolarization, and the like, there exists a certain amount of "static" cross-coupling of signals. In addition, such factors as Faraday rotation, spacecraft orientation and depolarization of signals due to rainfall effects, there is a rather large coupling of signals which is "dynamic" (i.e., time-varying) in nature. Both the static and dynamic cross-polarization interference act to seriously degrade the performance of frequency reuse systems. Recently, an article appearing in The Bell System Technical Journal, vol. 50, No. 9, November, 1971, at pgs. 3063 to 3069 by T. S. Chu entitled "Restoring the Orthogonality of Two Polarizations in Radio Communication Systems, I" suggested an orthogonalizing circuit which applies differential phase shift and differential attenuation to two signals to orthogonalize them. For a receiving system, the circuit described by Chu is implemented in the waveguide between the antenna and the receiver and it operates directly on the fields in a dual-polarized waveguide. It is designed to provide a single correction applied over the entire receive bandwith and its implementation on a channelized basis (i.e., separate corrections for different portions of the frequency band) would be difficult.